1. Field of the Invention
This invention relates to organosiloxane coating compositions. More particularly, this invention relates to thixotropic coating compositions wherein the film-forming material is a polyorganosiloxane that can be applied to selected areas of a substrate, particularly wafers of processed silicon, by screen printing.
2. Description of the Prior Art
The physical and electrical properties of polyorganosiloxanes comprising methyl- and phenyl substituted siloxane units make these materials desirable as passivation coatings for semiconductor devices such as transistors and integrated circuits. The coating are typically applied to selected areas of processed silicon wafers that are subsequently subdivided into a plurality of semiconductor devices. Once applied the coating must not flow beyond its initial borders to cover bonding pads or sites where test probes are subsequently placed in electrical contact with the device to measure its electrical properties.
Screen printing is a convenient means for applying passivation coatings to the etched silicon wafer. The coating composition must flow readily through the openings in the screen under the relatively low pressure applied by the squeegee or other device used to force the composition through the pattern of openings in the screen. The composition should adhere sufficiently to the surface of the wafer without becoming so "tacky" that it also adheres to the screen.
Coating compositions containing organosiloxane resins wherein silicon is bonded to hydrocarbon radicals including methyl and phenyl are known in the art. The compositions typically include the resin, a solvent for the resin and a filler such as silica. The composition may also include additives modify the properties of the coating composition or the cured resin.
Compositions containing resins with silicon-bonded hydroxyl and/or alkoxy groups in combination with organosilicon compounds containing functional groups such as alkoxy or silicon bonded hydrogen that are reactive with the groups present on the resin are disclosed in British Pat. No. 921,046, which issued on Mar. 13, 1963.
U.S. Pat. No. 4,395,443, which issued to Shimizu and Hosokowa on July 26, 1983 teaches film-forming compositions containing a resinous organosiloxane copolymer consisting essentially of R.sub.3 SiO.sub.0.5 and SiO.sub.2 units, where R represents a monovalent hydrocarbon radical, in combination with from 20 to 200 parts by weight per 100 parts of said resin of a hydroxyl terminated polydiorganosiloxane having a viscosity of from 10 to 2000 Pa.s at 25.degree. C. and a mixed organsiloxane/organic solvent.
U.S. Pat. No. 4,248,750, which issued to Murakami et al. on Feb. 3, 1981 discloses peelable films formed by reacting a vinyl-substituted polyorganosiloxane with a polyorganohydrogensiloxane. An alcohol- or polyether-modified polydiorganosiloxane is used to reduce the tendency of the material to accumulate static electricity and to improve the ability of the cured film to be coated with an adhesive.
Because of their tendency to flow after being applied, coating compositions comprising a solubilized organosiloxane copolymer containing units of the general formulae RSiO.sub.1.5 and R.sub.2 SiO where R represents a monovalent hydrocarbon radical, preferably a combination of methyl and phenyl radicals, and a silica filler typically cannot be used to apply screen printed coatings to selected area of semiconductor devices that border on bonding pad and test probe areas, which must remain free of coating.
U.S. Pat. No. 4,413,088, which issued to Frye on Nov. 1, 1983 discloses abrasion resistant coatings comprising (1) a dispersion of colloidal silica in a solubilized partial condensate of RSi(OH).sub.3, where at least 70 percent of the monovalent hydrocarbon radicals represented by R are methyl and, (2) as a flow control agent, from 2.5 to 15 percent, based on the weight of the composition, of a specified class of polysiloxane/polyether copolymers of the general formula ##STR1## where R and R' are monovalent hydrocarbon radicals; R" is lower alkyl, a has a value of at least 2, b has a value of 2 or 3, n has a value of 2, 3 or 4 and the value of x is at least 5. These copolymers function as flow control agents.
U.S. Pat. No. 4,477,626, which issued to Suzuki on Oct. 16, 1984 discloses thixotropic compositions comprising a vinyl-containing polyorganosiloxane. an organohydrogensiloxane containing at least two silicon bonded- hydrogen atoms per molecule and a platinum-containing hydrosilylation catalyst. The material responsible for the thixotropic character of the composition is a polyorganosiloxane containing at least 0.5 weight percent of hydroxyl groups that can be present as silanol groups or as silicon-bonded hydroxyalkyl groups, including polyoxyethylene and polyoxypropylene groups. Exemplified thixotropic agents include a hydroxyl-terminated polydimethylsiloxane containing an average of eight dimethylsiloxane units per molecule, a trimethylsiloxy-terminated dimethylsiloxane/3-hydroxypropylmethylsiloxane copolymer containing an average of fifty dimethylsiloxane units and seven 3-hydroxypropylmethylsiloxane units per molecule, and a trimethylsiloxy-terminated copolymer containing an average of thirty dimethylsiloxane units and three units of the formula ##STR2## per molecule.
The present inventors evaluated a number of hydroxyl-containing copolymers of the types disclosed in the aforementioned Frye and Suzuki patents and determined that many were unsuitable for one or more reasons.
One objective of this invention is to provide additives that impart thixotropic flow properties to silica-filled solubilized organosiloxane compositions without adversely affecting the ability of these materials to function as high resolution, screen printable compositions.
A second objective of this invention is to provide organosiloxane compositions capable of being screen printed onto a variety of substrates, particularly the silicon wafers used to fabricate semiconductor devices, without adversely affecting the electrical properties of these devices.